Study Indicates DTaP's Efficacy Wanes After Final Dose

According to a Sept. 13 article(www.nejm.org) in the New England Journal of Medicine, the ability of the diphtheria and tetanus toxoids and acellular pertussis adsorbed (DTaP) vaccine to fight off pertussis is relatively short-lived and diminishes sharply in the years following the fifth and final dose.

Outbreaks of whooping cough continue to occur regularly, with each successive outbreak recording an increase in peak incidence, so researchers from the Kaiser Permanente Vaccine Study Center set out to uncover why DTaP is not doing a better job. To answer the question, the researchers selected 4,281 children in Kaiser Permanente's Northern California health care system who had received a pertussis polymerase chain reaction (PCR) assay test result between January 2006 and June 2011. They assessed waning immunity after DTaP vaccination by comparing patients who were PCR-positive with two sets of controls. What they found was that protection against pertussis waned significantly during the five years after the last dose of DTaP.

"Increasing time since the fifth dose of DTaP was associated with an increasing percentage of positive PCR tests," the authors wrote. "The time since the fifth dose of DTaP was significantly longer for PCR-positive children than for PCR-negative controls, (while) case children received their fifth dose of DTaP significantly earlier than controls ... indicating that each year after the fifth dose of DTaP was associated with a 42 percent increased (risk) of acquiring pertussis."

The CDC currently recommends(5 page PDF) immunizing children at age 11 with the tetanus, diphtheria and acellular pertussis (Tdap) booster and vaccinating children as young as 7 in certain circumstances.

According to the study, DTaP's limited protection duration raises the question of whether the Tdap booster should be generally approved for children as young as eight. But before that can happen, several issues must be clarified, including Tdap's overall effectiveness and duration of protection.

"The findings suggest that whooping cough control measures may need to be reconsidered," lead author Nicola Klein, M.D., Ph.D., said in a news release(xnet.kp.org). "Prevention of future outbreaks may be best achieved by developing new pertussis-containing vaccines or reformulating current vaccines to provide long-lasting immunity. That said, the DTaP vaccine is effective and remains an important tool for protection against whooping cough for children and the communities in which they live, and following current CDC recommendations remains important."